In the last two decades, hydraulic vibrator devices have often been used instead of explosive seismic processes for transferring seismic energy into the earth in order, for example, to be able to research the subsurface characteristics of areas in which explosive seismic signal sources cannot be used or are undesirable. In order to inject seismic energy into the earth so that the signals can expand and provide reflections for analysis, the earth has been regarded, theoretically, as an elastic body. On this basis, it is necessary and sufficient to induce elastic stimulation of the subsoil in order to transfer seismic energy with as high a degree of efficiency as possible. For the purpose, vibrator plates are often attached to four-wheel drive trucks. These plates are driven by a vibrator drive signal which operates a hydraulic or other vibrating element coupled to the vibrator plate and the plate transfers the seismic energy to the soil.
With the aid of a lift system, the vibrator is positioned in its operating location against the ground. In order that the vibrator baseplate does not separate from the ground during vibration, it must be pressed down with a force which is larger than the force amplitude of the vibrator itself.
Because of the high pressures which appear near the baseplate of the vibrator, the soil compression often leads to a lasting deformation. In some cases, this deformation is initially desirable for reducing transference losses. These transference losses are caused by friction and transposition in the area of the baseplate. However, after the plate is positioned, only elastic deformation is desired.
When operating the vibrator on paved or high-surfaced ground, such as on streets, such prior placement or positioning is undesirable because it can lead to damage to the paved surface which should be avoided. Also, when the vibrator is operated and the soil is stimulated with increasing oscillating amplitude of the vibrator plate, a range of amplitudes can be achieved in which the elastic deformation gives way and leads to some plastic deformation of the subsoil which can also result in damage to the surface as well as to unpaved ground. The control of the change from elastic to plastic deformation of the subsoil is not as important to the degree of efficiency of seismic stimulation as it is to the avoidance of roadway damage.
Modern vibrators offer the possibility of measuring and monitoring the force imposed upon the ground. By using this dynamic force and in connection with the static load, the strain on the ground can be assessed. Regrettably, the permissible forces for any roadway structure cannot be provided precisely so that a determination of acceptable ranges which would allow protection against roadway damage with respect to safety factors and consistent with effective use of the vibrators for seismic purposes is, thus, not possible.